Question

Why the two outermost shell of transition elements are incomplete?

Answer 1

Elements of groups 3-12 of the periodic table are called the d-block elements as the d-orbitals get progressively filled with electrons in periods 4-6. The 4s, 5s, 6s and 7s orbitals have lower energies than 3d, 4d, 5d and 6d orbitals and are hence filled up before the 3d orbitals. (n-1) stands for the penultimate shell d-orbitals which can have 0-10 electrons while ns stands for the outermost shell which can have 1 or 2 electrons. Half and completely filled d-orbitals are more stable and preferrentially filled at the cost of the outermost shell.
Here are some electronic configurations;
Group3, period 4 Sc [Ar]3d14s2
Group5, period 4 V [Ar]3d34s2
Group6, period 4 Cr [Ar]3d54s1 (half-filled is more stable, so electron is present in 3d rather than 4s
Group7, period 4 Mn [Ar]3d54s2
Group8, period 4 Fe [Ar]3d64s2
Group11, period 4 Cu [Ar]3d104s1 Fully-filled are more stable
Group12, period 4 Zn [Ar]3d104s2
In the inner transition elements, the 4f (lanthanoids) and 5f (actinoids) orbitals are filled successively. This is the third outermost shell. They have a complete (n-1)d and ns shell. The half-filled and completely filled configurations are stable as in case of transition metals and an electron is is less in the (n-1)d shell to obtain t.he stable half or completely filled f shell.